The specular is a bit toned down where only ambient light is present. The "ambient world" light is supposed to represent pure dark or shadow in the game yet allow the player to navigate... so what would specular look like when almost completely dark? Perhaps there's a better method but this one looks quite good when BOTH real and ambient light hit anything.

Better options will be available when the Source Code is release of course. Hell, Sikkpin showed that you can do HL2 style Ambient Cubemap lighting (even now without the source code) but you'd have to replace all your existing light assets.